This invention relates generally to the field of distributed resources (DR) and to the stabilization control of small grid (microgrid) applications. This invention more specifically relates to a control arrangement for a DR such as a microturbine, fuel cell arrangement or the like, which includes a power-electronic interface to the grid. The control enables the DR to be operated in a manner that stabilizes voltage and current supplied to a load/grid and provides a means to transition from grid connected to grid independent operation.
Following load fluctuations and system faults, rotating machine dynamics are governed by the machine mechanical dynamics, sometimes resulting in poorly damped frequency and angle power swing oscillations. Dynamic loads can also cause voltage stability problems. This behavior may limit system loading and may detrimentally affect power supply characteristics.
To solve the problem mentioned above in a cost effective way, utilities have used supplementary controls to stabilize their systems. In particular, power system stabilizers (PSS) have become a widely accepted method of improving the small-signal stability of electric power systems that is applied to the field exciter of a synchronous generator.
There are many considerations in applying PSS""S, and each PSS must be tuned according to the particular conditions of the host system. Other efforts have also been made to design power swing damping controllers such as FACTS controllers for power systems. However, most of the efforts are focused on bulk power systems, not for isolated, smaller sized power systems or so called xe2x80x9cmicro-gridsxe2x80x9d such as those provided on islands or applications where the distributed resources (DR) are a significant percentage of the total generation.
U.S. Pat. No. 5,300,876 discloses an example of the above mentioned type of PSS. The content of this document is hereby incorporated by reference thereto.
Another desirable feature lacking in power-electronic based DR equipment is the ability to smoothly transition from grid connected to grid independent operation with the DR supplying a dedicated load. The ability to transition in this manner, without loss of power to the load, is imperative in critical load applications such as might be found in data centers and some process lines.
A first aspect of the invention resides in a power-electronic based DR (distributed resource) control arrangement which causes the DR to emulate a voltage source with a specified output impedance and stabilize the load power, the control arrangement receiving signals indicative of current (is) and voltage (xcexds) at an interface between the DR and the load, the control arrangement including circuitry which utilizes the signals indicative of interface current (is) and voltage (xcexds) and which: generates an internal frequency and internal angle references; compares internal frequency reference with a measured power frequency; uses the frequency difference to modulate power command for damping control of load power oscillations; and determines a current magnitude and phase angle for DR output utilizing the specified emulated impedance.
A second aspect of the invention resides in a control arrangement associated with a DR (distributed resource) for controlling the DR on the basis of a voltage measurement signal (xcexds) and a current measurement signal (is) from an interface between the load and the DR, the control arrangement comprising: a power control section which is responsive to the system voltage measurement signal (xcexds) and the system current measurement signals (is), which generates a system frequency signal ({tilde over (xcfx89)}s), a power measurement signal (Ps) and a system voltage signal (Vs) based on the system voltage measurement signal (xcexds) and system current measurement signals (is), and which compares the system frequency signal ({tilde over (xcfx89)}s) with a reference frequency signal ({tilde over (xcfx89)}*) and system power measurement signal (Ps) to produce an internal power angle signal (xcex4i); and a voltage control section which determines an error between the system voltage signal (Vs) with a reference voltage (v*) and produces an internal voltage signal (Vi).
A third aspect of the invention resides in a method of controlling a power electronic based DR (distributed resource) in a manner which emulates a voltage source behind an impedance and stabilizes power supplied to a load operatively associated with the DR, comprising: sensing system current (is) and system voltage (xcexds), at the output of the DR, and based on the one or more of the signals indicative of system current (is) and system voltage (xcexds) implementing the steps of: generating an internal frequency and internal angle references; comparing internal frequency reference with a measured power system frequency; using the frequency difference to modulate the power command for damping control of grid power oscillations; and determining, based on the specified impedance, a current and phase angle for DR output.
A further aspect of the invention resides in a method of controlling a DR (distributed resource) so as to behave as a power system stabilizer comprising the steps of: detecting a system voltage measurement signal (xcexds), a system current measurement signal (is) and computing the system power measurement signal (Ps) at the DR output; using the system voltage measurement signal (xcexds) and the system current measurement signal (is) to develop a system voltage signal (Vs); phase lock loop on system voltage signal (xcexds) to produce a system frequency signal ({tilde over (xcfx89)}s) and corresponding system frequency ramp signal (xcex3s); modifying the system frequency signal ({tilde over (xcfx89)}s) with a reference frequency signal({tilde over (xcfx89)}*) and the system power measurement signal (Ps) to generate a modified system frequency signal; frequency regulating the modified system frequency signal to develop a power reference signal (P*); generating a phase correction signal by phase shifting a signal based on a difference between the system frequency signal({tilde over (xcfx89)}s) and the reference frequency signal({tilde over (xcfx89)}*); modifying the power reference signal (P*) with the phase correction signal; power regulating the modified power reference signal to develop an internal power angle signal(xcex4i) and internal ramp signal (xcex3i); regulating a voltage error signal derived using a reference voltage and generating an internal voltage signal (Vi); using the internal voltage system (Vi), the internal power angle signal (xcex4i) and the system voltage control signal Vs to emulate a voltage source behind an impedance and to output a system current control signal (Is) and a system angle control signal (xcex8s) indicative of the angle between the system current control signal (Is) and the system voltage signal (Vs); modifying the system angle control signal (xcex8s) with the system frequency ramp signal (xcex3s) and internal ramp signal (xcex3i) to produce a modified system angle control signal (xcex3so); and outputting the modified system angle control signal and the system current control signal (Is) to the DR.
Yet another aspect of the invention reside in an apparatus for controlling a DR (distributed resource) so as to behave as a power system stabilizer, comprising: means for detecting a system voltage measurement signal (xcexds) and a system current measurement signal (is) and calculating a system power measurement signal (Ps) from the DR; means for using the system voltage measurement signal (xcexds) and the system current measurement signal (is) to develop a system voltage signal (Vs); means for on system voltage control signal (xcexds) to produce a system frequency signal ({tilde over (xcfx89)}s) and a system frequency ramp signal (xcex3s) indicative of the rotating angle of the system voltage (xcexds); means for modifying the system frequency signal ({tilde over (xcfx89)}s) with a reference frequency signal({tilde over (xcfx89)}*) and the system power measurement signal (Ps) to generate a modified system frequency signal; means for frequency regulating the modified system frequency signal to develop a power reference signal (P*); means for generating a phase correction signal by phase shifting a signal based on a difference between the system frequency signal ({tilde over (xcfx89)}s) and the reference frequency signal ({tilde over (xcfx89)}*); means for modifying the power reference signal (P*) with the phase correction signal; means for power regulating the modified power reference signal to develop an internal power angle signal (xcex4i) and internal ramp signal (xcex3i); means for regulating a voltage error signal derived using a reference voltage and generating an internal voltage signal (Vi); means for using the internal voltage system (Vi), the internal power angle signal (xcex4i) and the system voltage control signal Vs to emulate a voltage source behind an impedance and to output a system current control signal (Is) and a system angle control signal (xcex8s) indicative of the angle between the system current control signal (Is) and the system voltage signal (Vs); means for modifying the system angle control signal (xcex8s) with the phase angle ramp signal (xcex3s) and internal angle ramp signal (xcex3i) to produce a modified system angle control signal (xcex3so); and means for outputting the modified system angle control signal and the system current control signal (Is) to the DR.